The above mentioned filter bags for infusion products may have one or more chambers containing an infusion or extract product. Known filter bags may have a pickup tag connected to the filter bag by a connecting string.
The pickup tag and the filter bag may be attached to the connecting string, in known manner, by knotting, heat sealing, a heat-sealable stamp, or other suitable means.
String and pickup tag are suitably positioned on the filter bag thus formed (for example, wound around and attached to the filter bag, or tucked inside it) to form a product ready to be packaged in suitable containers or boxes. It is also known in the prior art that the filter bags may be wrapped in overwrap envelopes made by folding a sheet of overwrapping material into a U shape around the filter bag to produce a finished package.
Normally, the overwrap envelope is sealed in one of the following two different ways:
if the overwrap envelope has an opening flap, the two longitudinal sides of the overwrap envelope are sealed; or
if the envelope does not have an opening flap, two longitudinal sides and one transversal side of the overwrapping sheet are sealed (of particular relevance for this specification).
To make overwrap envelopes without opening flap, the prior art teaches the use of machines comprising a plurality of grippers, each of which holds a single bag and which are arranged radially on a rotary carousel.
Following a rotation of the carousel, the filter bags are carried close to operating stations where the string and pickup tag are applied.
A further rotation of the carousel brings the filter bag close to a station where a continuous web of overwrapping material is fed. The sheet of overwrapping material used to make the overwrap envelope for a respective filter bag is cut from this web.
In the proximity of this feed station, the filter bag is moved radially away from the carousel through the agency of grippers in order to enable the sheet to be fed between the carousel grippers and the filter bag itself. The filter bag is then moved towards the carousel grippers again so that the sheet is folded into a U shape round the filter bag.
Next, the filter bag with the sheet of overwrapping material folded into a U around it is carried to a first sealing station, equipped with a first sealer, which makes a first set of sealing lines along the transversal side, or top, of the sheet. The transversal lines of the first set are straight and transversal to a direction of longitudinal extension, or, in other words, parallel to the transversal side of the overwrap envelope.
A further rotation of the carousel brings the partly sealed package to a pair of stretching rollers which move the partly sealed package away from the carousel towards a drum provided with recesses, each recess receiving a single package.
Following a rotation of the drum, the package, located inside a recess, is carried to a second sealing station equipped with a second sealer, which makes a second set of sealing lines along the longitudinal sides of the sheet. The sealing lines of the second set are straight and parallel to the direction of longitudinal extension of the overwrap envelope, that is to say, perpendicular to the lines of the first set of sealing lines.
Thus, at the zones of contact between the longitudinal sides and the transversal side, the overwrap envelopes just described have sealing lines of the first set which intersect sealing lines of the second set. In other words, the corner zones, where the longitudinal sides and the transversal side meet, are sealed twice. In these zones, the overwrapping material may thus be damaged and weakened.
Moreover, the machines which seal overwrap envelopes in two steps have some disadvantages. They are relatively expensive because they have two sealing stations and require very precise conveying devices to transport each package from the first sealing station to the second.
Alternatively to machines of this kind which perform sealing in two steps, machines are also known which can seal the longitudinal sides and the transversal side of the sheet of overwrapping material in a single step using a sealing gripper which is suitably shaped on three sides (and which is mounted directly on the aforementioned carousel). More specifically, the sealing gripper has sealing profiles or impressions, all parallel to each other and parallel to the direction of longitudinal extension of the overwrap envelope, which make corresponding sealing lines on the longitudinal and transversal sides of the overwrap envelope. As a result, on the transversal side of the envelope, the sealing lines are much shorter than the sealing lines on the longitudinal sides. The sealing lines on the transversal side of the envelope cannot, however, be shorter than a minimum length which guarantees the strength of the overwrap envelope. To prevent loss of seal strength, it is therefore necessary to use relatively long sheets of overwrapping material, longer than needed to contain the filter bag (unsealed). That means increased costs in terms of material used.
Similarly, it is also necessary to use relatively large sealers in order to make sealing lines which are long enough to guarantee seal strength.